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Q & A: Evaporative Cooling

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Most recent answer: 10/22/2007
Q:
Do liquids cool as they evaporate?
- Theresa
Oaktree, Goodrich, MI
A:

Theresa -

When you heat up a liquid, it evaporates and turns into a gas. But it takes energy to pull a molecule out of the liquid and turn it into a gas, since the liquid molecules stick together and it's kind of hard to pull them apart. That leaves a little less energy for keeping all the molecules jiggling around. In other words, they actually do cool down a little bit just as they evaporate. So, left to itself, the liquid would cool down as it evaporates.

This doesn't mean that steam is cooler than water, though. If you gradually raise the temperature of a pot of water, the water boils off and turns into a gas (steam). Up to a certain temperature, it is a liquid. When it reaches the boiling point, adding more heat just boils off more of the liquid, not raising the temperature. After the liquid has boiled off, as you add more heat the gas can get a lot hotter than it could as a liquid.

Evaporating liquids can cool other things, too. For instance, "evaporative cooling" is a system that is often used to cool small buildings. A fan blows the warm air in the building through a screen covered with cold water. As the warm air goes through, it heats up the water, causing it to evaporate. The heat goes from the air into the water, so the air gets cooler. A similar thing happens when a liquid sits on your skin (say as sweat). Heat will then flow from your skin into the liquid, evaporating the sweat. So you cool down.

-Tamara + Mike W.


(published on 10/22/2007)

Follow-Up #1: cooling liquids

Q:
do liquids cool as they evaporate
- Anonymous
A:
Yes, unless something else is pouring heat into them.
The reason is that it takes some energy to pull a molecule loose from the liquid, and that energy is then lost from all the thermal jigglings, so things are cooler.

Mike W.

(published on 10/22/2007)

Follow-Up #2: cooling by evaporation

Q:
Why do liquids cool as they evaporate?
- ~ SwEeT ~ (age 13)
Perth, W.A
A:
For something to be 'hot' means that it has lots of random jiggles in its components, with lots of energy. The liquid molecules also have less energy than gas molecules just because they're stuck together, the same way a rock has less energy when it's sitting on the Earth than when it's pulled away. So when molecules leave the liquid they need to get that energy to pull away from somewhere. It's taken away from the random jiggles, leaving everything a little cooler.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.